Section 1.10 Constructing Groundwater Flow Nets
Much of this section is based on Brassington (1990: Chapter 6)
Groundwater level data can be used to determine the direction of groundwater flow by
constructing water table contour maps and flow nets. are It is necessary to have a
minimum of three observation points to calculate a flow direction.
1. Relate groundwater field levels to a common datum. A map datum is best with
position accurately plotted on to a scale plan (Figure 1.10.1)
2. Draw a pencil line between each of the observation points and divide each line
into a number of short, equal lengths in proportion to the difference in elevation at
each end of the line. In Figure 1.10.1, each division in the lines A – B and B – C
is 0.2 m, while on line A – C each division is 0.1 m. The length of each division
on the plan is not related to the scale of the plan.
3. Join points of equal height on each of the lines to form contour lines. Select a
contour interval which is appropriate to the ovedrall variation in water levels in
the study area. The direction of groundwater flow is at right angles to the contour
lines.
Source: Brassington (1990)
Note: The water table elevation values must be related to the same datum.
Figure 1.10.1. Estimation of groundwater contours and flow directions, based on
water levels in three observation boreholes.
This procedure may be applied to a much larger number of water level values to
construct a contour map of the groundwater table as in Figure 1.10.2.
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Source: Brassington (1990)
Figure 1.10.2. Construction of a groundwater contour map and flow net.
(a) Values of groundwater levels are indicated on a plan, and key contours are plotted
using the techniques shown in Figure 1.10.1. In this example contours at 22 m and
26 m were used.
(b) The remaining contours are interpolated in the same way. Flow lines were
sketched in, perpendicular to the contour lines, starting on the 30 m contour at a
spacing of 500 m.
1. Locate the position of each observation point on a base map of suitable scale and
write the water level against each position.
2. Study these water level values to decide which contour lines would cross the
centre of the map. Select one or two key contours to draw in first.
3. In Figure 1.10.2a the 22 m contour was drawn by interpolating its position
between each pair of field values, following the procedure illustrated in Figure
1.10.1.
4. Once this was completed the 26 m contour line was drawn in the same way.
5. The remaining contours were drawn using the two key contours as a guide, in
addition to interpolation between the field values.
6. Once the contour map is complete, flow lines can be drawn by first dividing a
selected contour line into equal lengths.
7. Flow lines are drawn at right angles from this contour, at each point marked on it.
8. The flow lines are extended until the next contour line is intercepted, and are then
continued at right angles to this new contour line.
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9. In Figure 1.10.2 contour map the 30 m contour was divided into 500 m intervals
as the starting point for constructing the flow lines.
10. Always select a contour that will enable you to draw the flow lines in a downslope direction.
It is important to bear in mind what you have already learned about a particular aquifer
when you are constructing a water table contour map and flow net. Information on the
geological structure and variation in aquifer hydraulic properties should be taken into
account. This is particularly important where groundwater flow is through fissures,
karstic limestones being an extreme example. It is not unusual in the Mountain Aquifer,
for example, to find that the fissure systems do not permit flow in the direction indicated
by the contours, and real flow direction can only be deduced by using tracers. Compare
the groundwater flow directions in Figure 1.3.3 with the water table contours of Figure
1.3.4.
A water-table contour and flow-line map represents groundwater movement in plan form
only. This is only part of what is actually taking place in three dimensions. When viewed
in cross-section, groundwater flow paths curve towards a discharge point such as a
spring-line, stream or a pumping well. Figure 1.10.3 shows two examples of groundwater
flow at depth in an aquifer. This three dimensional conceptualisation of groundwater flow
in an aquifer is very important in understanding groundwater flow dynamics and the
limitations of contour maps and flow nets.
Source: Brassington (1990)
Figure 1.10.3. Changes in groundwater levels can be easily studied if you construct a
water level changes map. In the above example, water table contour maps (a) and (b),
covering the same area in successive years are superimposed. The difference in values are
noted at points where the contours cross (c), and new contours are drawn to show the
amount of change (d). Water-level change maps are useful to measure the local effects of
recharge and discharge. In the above example the recharge area lies in the southwest
corner and the discharge area is in the northeast, facts that are not obvious from the water
table contours but become apparent once the water level change map is constructed.
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The spacing of water table contour lines gives a good indication of variation in aquifer
permeability values. Where contours are close together it is indicative of low
permeabilities, because a steep hydraulic gradient is needed to ‘push’ the water through
the aquifer. Where groundwater contours are more widely spaced, the converse is true,
and the aquifer is likely to be much more permeable.
Groundwater flow lines indicate not only the overall direction of flow but where flow is
concentrating. This is also an indicator of variations in aquifer permeability. In the
example of 1.10.2b, the groundwater contour spacing suggests that the south-west of the
area of the aquifer is more permeable than the north-east. The flow lines show that there
is a concentration of flow in the area marked ‘A’, and this is probably an area of
discharge to surface streams. If this were to be used to select favourable locations for a
new well, area ‘A’ would have much to commend it as it is both in an area of high
permeability values and where flow lines converge.
Water-level change maps (Figure 1.10.3) are used to calculate changes in the volume of
water stored in an aquifer as part of a water balance exercise (see Section 1.4). These
maps are also useful when assessing the local effects of recharge and abstraction.
The hydrologist needs to make use of data from field observations to complete and check
contour maps and flow nets. Observation borehole records can be supplemented by using
information on the position and elevation of springs. Great care has to be taken, though,
to ensure that these spring discharge from the same aquifer as is penetrated by the
borehole. If the spring discharges from the same geological formation it probably belongs
to the same aquifer. If the water chemistry is the same in the springs and as extracted
from the borehole, this too would suggest the same aquifer source.
In addition to springs, streams and rivers can also be used to help construct a groundwater
contour map. Field evidence is again needed, and this may include outcrops of the aquifer
in the stream bed, and a significant increase in flow as the stream flows across the
aquifer. Other information commonly used includes topographic details, usually taken
from published maps. In Gaza it is usual to assume that the coastline approximates to the
zero groundwater contour line. This applies not just to the coastal aquifer but also to the
western basins of the Mountain Aquifer. Similar to surface water, groundwater follows a
profile with the coastline as its base level. The Dead Sea acts as the surface/groundwater
base level for the eastern basins of the Mountain Aquifer.
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